Caroline M Burberry, David Cannon, Terry Engelder, John Cosgrove

The Teton anticline and adjacent structures at the deformation front of the Sawtooth Range fold-thrust belt in Montana are fractured in such a way that some have used these as a model for the fractures that propagate during buckle folding. However, advances in understanding both the process of folding in forelands and the evolution of fracture patterns found within these folds suggests that it is time to reinterpret the nexus between fracturing and folding within this classic structure. Data gathering includes two steps starting with the documentation of the along-strike changes in the geometry of the Teton anticlines, using field measurements, aerial photographs and seismic lines, and then the collection of fracture orientation data and abutting relationships. With the benefit of seismic lines, Teton anticline is best described as a fault-related fold, forming in response to changes in the underlying ramp and flat geometries. The propagation of joints (mode I cracks) initiated with formation of two major sets whose orientation is controlled by pre-folding, regional stresses. Two more joint sets propagated in local stress fields, developed in response to the growth of fault-related folds. Some early joints were reactivated as wrench faults during amplification and tightening of the anticlines. The fracture sets identified in this study are consistent with two unrelated mechanisms starting with propagation in a regional stress field. This first mechanism is poorly understood but might be related to regional stretching in the Sawtooth Range orocline. The second mechanism is associated with tangential longitudinal strain during anticline development. Thus, we suggest that fracture networks across folded hydrocarbon reservoirs should be interpreted and characterized in the light of a more complex tectonic setting and the genetic or geologic history of the system as a whole.